1. Field of the Invention
The present invention relates to a transfer apparatus for use with standardized mechanical interface (SMIF) systems for facilitating semiconductor wafer fabrication, and in particular to an interface and transfer apparatus which may be configured either as an indexer which separates a workpiece carrying cassette from a storage and transport pod by lowering the pod door and cassette away from the stationary pod shell, or as a load port opener which separates the cassette from the pod by raising the pod shell away from the stationary pod door and cassette.
2. Description of the Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 0.5 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) a minienvironment supplied with ultraclean air flows surrounding cassette load ports and wafer processing areas of processing stations so that the environments inside the pods and minienvironment become miniature clean spaces, and (3) robotic transfer assemblies to load/unload wafer cassettes and/or wafers from the sealed pods to the processing equipment without contamination of the wafers in the wafer cassette from external environments. The system provides a continuous, ultraclean environment for the wafers as they move through the wafer fab.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called "bottom opening" pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide "front opening" pods, in which the pod door is vertically oriented, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted in the pod shell.
In bottom opening configurations, in order to transfer a wafer cassette from within the SMIF pod to within a particular processing station, a pod is loaded either manually or automatedly onto a load port affixed to a front end of the process tool. For ergonomic reasons, Semiconductor Equipment and Materials International ("SEMI") has set a standard load height for the upper surface of the load port of approximately 900 mm from the ground. The upper surface of a load port is defined by a port plate including a central opening, and a port door, covering the central opening when no pod is present on the load port.
The pod is designed so that, when loaded onto the load port, the pod door overlies the port door, and the pod shell overlies the port plate. Once on the load port, mechanisms within the port door decouple the pod door from the pod shell so that the pod door and pod shell are supported on the port door and port plate, respectively. The port door and the port plate thereafter translated away from each other to separate the pod door from the pod shell to allow access to the cassette. A pick and place robot, such as for example a cylindrical body and arm robot, then transfers the cassette between the load port minienvironment and the process tool. Alternatively, the pick and place robot may include an end effector capable of gripping individual workpieces for transfer between the cassette and process tool.
In one type of load port configuration, referred to as an indexer, the port door is capable of vertical translation while the port plate remains in a fixed position at the load height (e.g., at 900 mm). In this configuration, after the pod is loaded onto the load port and the pod door and shell are decoupled from each other, the port door, with the pod door and cassette supported thereon, is lowered while the port plate and pod shell remain stationary at the load height. The cassette is lowered into a minienvironment beneath the port plate, from which the cassette and/or wafers may be transferred to and from the process tool. The port door is coupled to an elevator for translating the port door away from and back to the port plate.
In a second type of load port configuration, referred to as a load port opener, the port plate is capable of vertical translation while the port door remains in a fixed position at the load height. In this configuration, after the pod has been loaded onto the load port and the pod door and shell are decoupled, the port plate, with the pod shell supported thereon raise upward, while the port door, pod door and cassette remain stationary at the load height. A minienvironment is provided around the cassette as the port plate moves upward, from which minienvironment, the cassette and/or wafers may be transferred to and from the process tool. The port plate is coupled to an elevator for translating the port door away from and back to the port plate. An example of such a load port opener is disclosed in U.S. patent application Ser. No. 08/730,643, entitled "Load Port Opener", previously incorporated by reference.
Each load port configuration conventionally used to interface a SMIF pod with a process tools requires a significant investment of time and money to properly develop and maintain. For example, each load port used in a wafer fab requires a separate design concept, component design and selection, component qualification and procurement, prototype assembly, product testing and optimization, product certification, product documentation, inventory, manufacturing training and field service training.
The type of load port used in a wafer fab is dependent on the particular process tool configuration and requirements, and it is generally more advantageous to use one load port configuration over another. For example, in some process tools, there may not be sufficient space below the 900 mm load height for an indexer to lower the wafer cassette and then transfer the cassette and/or wafers between the load port and process tool. For such configurations, a load port opener configuration would be used. Alternatively, it may advantageous to use an indexer configuration because the lowering of the port door and cassette positions the wafers for transfer into the process tool at a relatively low height, and the wafer handling robot used to transfer the wafers need have only a relatively small z-axis stroke.
As such, semiconductor handling equipment manufacturers must provide both types of load port configurations. At present, the two separate configurations require two separate designs, and two separate processes for the manufacturing, testing, certification, documentation and maintenance of the load ports. Additionally, separate spare and replacement parts need to be stocked and supplied as needed for the separate configurations.